FP transport experiments at VTT

Phebus FP Programme highlighted some important uncertainties in the transport of especially volatile fission products within the primary circuit during a severe nuclear accident. It was shown that changes in thermal hydraulic and chemical conditions may lead to revaporisation of deposited fission product compounds and their delayed release to the containment. Also the fraction of gaseous iodine found to be released from the circuit was unexpected. A detailed study on the transport of volatile fission products was initiated at VTT during the 4th Framework programme project Revaporisation. Separate effect tests were carried out in order to determine the release of caesium compounds in temperature and flow conditions relevant to severe accidents. During the project a similar facility was constructed at JRC/ITU, where real deposited fission product samples from Phebus FP tests have been studied since then. Since 2009 the effects of reactions on primary circuit surfaces on the transport of gaseous iodine have been conducted using EXSI PC facility at VTT. Thus far samples containing iodine, caesium, molybdenum, boron and silver have been treated in a mixture of argon, steam and hydrogen in temperature between 400 and 650 °C. The tests have enabled to determine the fraction of iodine and other fission products released in gaseous and in aerosol form. The aim of the current test programme is to verify the main factors influencing the gaseous iodine formation within the primary circuit. A further objective is to propose a model for the reactions on primary circuit surfaces that could be incorporated into severe accident codes